What is the difference between artificial intelligence and swarm intelligence?
Swarm intelligence (SI) is a sub-field of or an approach to artificial intelligence (AI), where you have multiple individuals (for example, artificial ants), which collectively can produce what we (or most of us) would intuitively call intelligent behaviour.
SI is sometimes categorized as a sub-field of evolutionary computation (which also includes evolutionary algorithms, such as genetic algorithms, genetic programming, evolution strategies, and so on), which is often considered a sub-field of AI or techniques to produce artificial intelligence, because all these techniques are often based on the use of multiple individuals/solutions (that either compete or collaborate with each other).
One of the most commonly used SI techniques is ant colony optimization algorithms (proposed by M. Dorigo and further developed by other people like Luca M. Gambardella), which have been successfully applied to solve the non-decision version of the NP-complete problem (in simple words, it's a combinatorial problem that may require exponential time to be solved in the usual case) known as the travelling salesman problem. There are other SI techniques, which are somehow similar to ACO algorithms, such as particle swarm optimization or the artificial bee colony algorithm.
Occasionally, SI may also be categorised as a sub-field of computational intelligence, which often refers to specific techniques to create artificially intelligent systems (i.e. programs that exhibit what we would call intelligence) that are more based on or inspired by the biology, such as neural networks, genetic algorithms, or, in fact, SI algorithms, such as ACO algorithms. However, CI can also be considered a sub-field of AI, given that it studies techniques to produce artificial intelligence, so, in the end, as I said above, SI is an approach to AI, which includes other approaches, such as evolutionary algorithms, rule-based systems, deep learning or other machine learning techniques.